8 characters, 4 of which are static between your passwords and 4 of which any reasonably clever person can connect with the service name, in a predictable pattern, appears to be roughly as secure as using the same weak password on any service.

8 characters is well below the threshold of brute-force attacks these days, and once I've got one, and I see it has the site name in it, I'm gonna catch on to the game and there goes your other sites' passwords.

Me too :-)